The initial line in the San Diego Trolley system, the Blue Line first opened between Centre City San Diego and San Ysidro on July 26, 1981, at a cost of $86 million (equivalent to $ million in ), using the existing tracks of the
San Diego and Arizona Eastern Railway, which the Metropolitan Transit Development Board had purchased from
Southern Pacific on August 20, 1979, for $18.1 million (equivalent to $ million in ). In 1986, the line was named the
South Line to differentiate it from the new
East Line to
Euclid Avenue, which later became the
Orange Line. On July 2, 1992, the line was extended north from downtown with the opening of
County Center/Little Italy station. Blue Line trains traveled between America Plaza to San Ysidro on a regular 15 minute frequency, with a minute frequency during weekday rush hours. Blue Line service to Old Town was reinstated once the Mid-Coast trolley extension to UC San Diego was completed. UC San Diego Health paid $30 million to rename the route the
UC San Diego Blue Line in 2015.
Trolley Renewal Project To accommodate the new
Siemens S70 models and allow for their use on the line, the Blue Line stations needed to undergo renovation, although this was done over a period of five years to prevent the disruption of operation of the trolley. The project to renovate the stations, called the Trolley Renewal Project, began in fall 2010. After beginning operation on the Green Line in 2011 and on the Orange Line in 2013,
low-floor Siemens S70 LRVs began operation on the Blue Line on January 27, 2015. Renovation of rail track and the final eleven Blue Line stations was completed in late 2015.
Mid-Coast extension The
Mid-Coast Corridor Transit Project was an Ridership is projected at 34,700 trips in 2030. The extension primarily follows the right-of-way of the
Coaster and
Interstate 5, with an elevated deviation around the UCSD area. MTS estimated construction costs of $2.1 billion. Testing of the line began in June 2021, and it opened to revenue service on November 21, 2021, The Mid-Coast Corridor Transit Project was done in conjunction with the
North Coast Corridor project, which upgrades the
LOSSAN Corridor further to the north. Both projects build upon the original right-of-way of the
Surf Line, which was built in the 1880s as the original railway from Los Angeles to San Diego.
Overnight Express bus On January 26, 2025, the MTS established
bus route 910, an "
Overnight Express" bus operating on the southern section of the
Blue Line from midnight to 5 A.M., when the Blue Line isn't running. Route 910 operates between and
San Ysidro Transit Center, with intermediate stops at the , , , , and transit centers. == Stations ==